(1) Field of the Invention
The present invention relates to asphaltene-containing liquid hydrocarbons and, more particularly, to measurement and/or control of the agglomeration of asphaltenes in hydrocarbon liquids.
(2) Description of the Related Art
Asphaltenes are organic heterocyclic macromolecules which occur in crude oils. Under normal reservoir conditions, asphaltenes are usually stabilized in the crude oil dispersion by maltenes and resins that are chemically compatible with asphaltenes, but that have much lower molecular weight. Polar regions of the maltenes and resins surround the asphaltene while non-polar regions are attracted to the oil phase. Thus, these molecules act as surfactants and result in stabilizing the asphaltenes in the crude. However, changes in pressure, temperature or concentration of the crude oil can alter the stability of the dispersion and increase the tendency of the asphaltenes to agglomerate into larger particles. As these asphaltene agglomerates grow, so does their tendency to precipitate.
Precipitation of asphaltenes in crude oil or in process streams of oil is economically costly because of lost production and maintenance required to clear blockages caused by the solid materials.
Various methods have been devised to minimize asphaltene precipitation. For example, pressure and temperature conditions can be maintained, chemical stabilizers may be added to mimic and to enhance the stabilizing affect of the natural resins and maltenes, or devices such as magnetic flux assemblies described in U.S. Pat. No. 5,453,188 may be used. While methods that minimize asphaltene precipitation can result in significant economies, they have been hampered by a lack of a method for measuring and monitoring the agglomerative state of the asphaltenes in a particular stream at a particular time. Without knowing the agglomerative state of the asphaltenes in the stream, it is unclear when or how much to treat the liquids to prevent asphaltene precipitation.
Conventional methods for determining the size and concentration of asphaltene particles in hydrocarbons, such as those described in U.S. Pat. No. 4,238,451, or in Standard Method IP 143/84, require sampling, transport to a laboratory and testing by precipitation and filtration, centrifugation, titration with a destabilizing solvent, or other time-consuming techniques. Thus, these methods are not suitable for real-time, on-line monitoring of agglomeration.
Although methods for testing for the size and concentration of particles in optically clear streams have been modified and applied to hydrocarbons, many have been much less successful in crude oil and other in-process oil streams due to fouling and opacity. For example, the optical system of Yamazoe, et al., U.S. Pat. No. 4,843,247, measures asphaltene content, but provides a washing means to remove the sample solution from the optical probes each time a sample measurement is carried out. Such washing requires more complex measuring devices and infers that fouling over time may hinder the accuracy of the optical measurement.
Direct centrifugation of crude measures the total amount of asphaltene present, but provides no information on degree of agglomeration of the particles or their tendency to remain in a stable dispersion.
Non-optical tests have recently shown promise in measuring particle characteristics. Anfindsen et al., U.S. Pat. No. 5,420,040, correlated the precipitation of asphaltenes in oil with changes in conductance or capacitance. However, this method requires transferring a sample of the liquid to be measured to a measuring cell and is not carried out on-line. Furthermore, the process is carried out stepwise and cannot be done substantially instantaneously since a time delay is required to allow for the precipitation of asphaltenes to occur.
Behrman and Larson, MBAA Technical Quarterly, 24, 72-76 (1987), describe on-line monitoring of particles over 0.8 microns in brewery streams by the use of an ultrasonic monitoring device. They use a piezoelectric transducer to generate an acoustic signal and to detect acoustic energy resulting from scattering from particles in the liquid streams. The device permitted on-line, real time measurement of particle concentrations, but did not permit the measurement of particle sizes or a particle size distribution.
More recently, Lin, et al., "Neutron Scattering Characterization of Asphaltene Particles", presented at ACS National Meeting, San Francisco, Calif., April 1997, reported the use of small-angle neutron scattering (SANS) to determine the size and concentration of asphaltene particles in dilute solution in 1-methyl-naphthalene-D10. The study concentrated on small, "basic", asphaltene particles and reported that larger particles, which might be important to macroscopic properties, could not be measured by today's small angle scattering instruments and would be very difficult, if not impossible, to measure with light scattering methods.
de Boer, et al., SPE Production & Facilities, pp.5-61, February (1995), report the investigation of asphalt precipitation in oils and describe using back-scattered energy from an acoustic probe to sense asphaltene particles. A multi-channel analyzer was used to sort the signals into two amplitude classes corresponding to small and large particle sizes. The acoustic sensing procedure was used to monitor the relative numbers of large and small particles during heptane titration of the oil to induce asphalt precipitation. Since the method required the addition of significant amounts of n-heptane to the oil, it would be impractical to apply the test to an in-process stream and on a real-time basis. However, without the addition of n-heptane to initiate precipitation, the scattered acoustic energy measurements of the method taught by deBoer, et al. would be meaningless.
Later, a group from the same laboratory reported the laboratory use of the same ultrasonic particle analyzer to study the utility of asphaltene inhibitors. (Bouts, et al., J. Petr. Tech., 782-787, September, 1995). The method included a test cell attached to a sonic probe which acted as described above to measure the energy scattered from particles in the liquid. A multichannel analyzer counted particles and sorted the detected scattered energy into thirteen amplitude classes. The two channels measuring the smallest particles and the remaining 11 channels measuring larger particles were respectively lumped together to monitor "small" and "large" particles versus time as the sample was titrated with n-heptane to destabilize the asphaltenes. The purpose of the method was to test various inhibitors by monitoring the formation of asphaltene agglomerates as a function of the heptane added and the inhibitor content. The study did not show how to interpret or use particle size distribution data for more than two particle size ranges. Furthermore, the article did not disclose how the agglomerative state of asphaltene particles in a hydrocarbon liquid could be determined on a real-time basis, without dilution, or without removing a sample of the liquid from the stream or tank in which it is contained.
Thus, despite progress with promising techniques in related areas, a suitable method has not been available to measure the agglomerative state of asphaltenes in oils, such as crude oil or any other optically opaque hydrocarbon liquid, on a real-time basis and without removing a sample from the process stream. The lack of such method has also limited the ability to control the agglomeration of asphaltenes in such oils.